Automatic levels are well known in the construction industry. They are tripod mounted surveying instruments that need not be precisely leveled to obtain a precise level line of sight, and temperature changes or ground settling which would throw ordinary levels out of level are of no consequence to an "automatic". However, these instruments are rather costly and delicate. The compensators used have relatively heavy gravity sensing devices made up of a system of prisms and/or mirrors which are suspended by fine wires or frictionless high quality ball bearings. These devices also require close fitting damping devices to stabilize the delicately hung optics. The compensators in use today correct the line of sight by deflecting the target image relative to a fixed crosshair type index line.
To date no one has come up with a simple, low cost, rugged compensator which can be used on inexpensive low power and relatively lower precision instruments such as a hand level. A hand level is a hand held sighting instrument, usually of unit power, used to establish a horizontal or level reference line of sight. Hand levels in common use today are of the "Locke" type which is basically an open tube with a flat cover glass at the objective end and a peep sight at the eye end. A spirit level is mounted over a slot on the upper side of the tube, a crosswire is mounted centered below the spirit level crosswise to the slot and a right angle prism is mounted in the tube below the spirit level and crosswire to reflect an image of the level and the crosswire toward the eye end. A convex lens which has been diametrically cut in half is mounted in the tube between the prism and the peep sight so that the lens is not being used when the target area is being viewed but the lens is used when the spirit level and the crosswire are being viewed. When sighting through the peep sight, part of the field of view is an open sight to the target area and immediately adjacent to it and visible at the same time is a sharply focused view of the image of the spirit level and the crosswire. Using the hand level requires that the instrument be aimed at the target so that the target is seen in the open sight portion of the instrument near the bubble image. The instrument is then leveled by tipping it until the crosswire exactly bisects the bubble. While holding the instrument level, the end of the crosswire is used as the index to obtain the level reading on the target being viewed.
Some Locke type hand levels incorporate Galilean telescopes to magnify the target about two to three times for better discrimination. The Abney level is similar to the Locke type and is used the same way for leveling. In the Abney level the spirit level is pivotally mounted above the sighting tube by means of a support having a graduated arc. To measure or set a grade, the spirit level is rotated to an angle or percent grade as defined by the graduated arc and then the instrument is sighted normally. When the bubble bisects the crosswire the line of sight of the instrument is inclined to the horizontal to the amount set.
Some tilting levels utilize the principle of viewing the target area and the spirit level simultaneously through a common eyepiece. These instruments use erecting type telescopes with a crosshair type reticle in the target viewing area and are usually in 28X magnification area for extremely accurate work. The optical train used to view the bubble is complicated and the bubble is split lengthwise optically, and one end of the bubble is reversed and brought in line with the other end of the bubble. This is called a coincidence type bubble and in use the instrument is set up on a tripod or other firm support and the telescope is fine leveled by means of a micrometer type knob. When the two images of the opposite ends of the bubble are in coincidence, the instrument is level and the level reading is made utilizing the crosshairs.